feat(desktop): add local device pairing - #49291

Summary

• Add desktop Settings → Pairing and a Pair device command with a local-network QR code and copyable direct connection link.
• Keep an optional persisted Keep screen active setting for the desktop app.
• Encode credentials in the direct server URL fragment so they are not sent in the initial HTTP request.
• Load /connect before server connections and health checks so pairing credentials are saved before API requests.
• Serve the web shell and assets before authentication while keeping API and OpenAPI requests authenticated.
• Use a compact, theme-aware QR dialog with visible titles, copy feedback, and responsive sizing.

Pairing URL

http://<server>/connect#<base64url({ username: "opencode", password })>

The URL points straight at the advertised OpenCode server. The /connect route derives the server address from its own origin and stores the fragment-carried credentials before authenticated requests begin.

Validation

• App pairing URL tests — 6 passed.
• App typecheck — passed.
• Desktop typecheck — passed.
• Desktop migration registry check — passed.
• CLI web-shell/authentication tests — 2 passed.